Reference voltage generating circuit with controllable linear temperature coefficient

ABSTRACT

A voltage divider circuit is connected between the output terminals of a constant-voltage power supply outputting a constant voltage. A constant-current source varies linearly, relative to temperature, the current level flowing to or from the voltage divider junction of the voltage divider circuit. The constant-current source comprises a first transistor and a second transistor connected to a current mirror circuit, and a resistor connected between the ground and the emitter of the second transistor. The base of the current extracting transistor is connected to the bases of the first transistor and the second transistor, and the collector and emitter are connected between the respective voltage divider junction and ground to obtain a current from the voltage divider junction. A current proportional to temperatures and inversely proportional to the value of the resistor can thereby be obtained from the voltage divider junction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a reference voltage generator in acharging device.

2. Description of Related Art

Charging devices for charging batteries generally comprise an internalreference voltage generator for comparing the battery voltage with thereference voltage output by the reference voltage generating circuit,and controlling battery charging according to the detected voltagedifference. Because the optimum charging voltage of the battery variesaccording to the temperature, the reference voltage generating circuitis built with output temperature characteristics matching the batterycharacteristics to achieve optimum control in charging devices thatmaintain a constant correlation between the ambient temperature andbattery.

An example of a reference voltage generating circuit known in the priorart is shown in FIG. 1. This reference voltage generating circuitcomprises resistors R₅₁ and R₅₂, n (where n is an integer) diodesD₁˜D_(n), and resistors R₅₃ connected in series in this order betweenthe constant-voltage power supply 1, which outputs a voltage with littletemperature-dependent variation, and ground, and outputs the referencevoltage V₀ from between the ground and the output terminal 3 connectedto the junction point between resistor R₅₁ and resistor R₅₂.

The reference voltage V₀ can be expressed by Equation (1) where thevoltage of the constant-voltage power supply 1 is V_(cc), the forwardvoltage of diodes D₁˜D_(n) is V_(F), and the current flowing throughresistors R₅₁ and R₅₂, n (where n is an integer) diodes D₁˜D_(n), andresistor R₅₃ is I₅₀.

V₀=V_(cc)−I₅₀×R₅₁

 =(R₅₂+R₅₃)V_(cc)/(R₅₁+R₅₂+R₅₃)+n R₅₁V_(F)/(R₅₁+R₅₂+R₅₃)[V]  (1)

The temperature characteristic ∂V₀/∂T of the reference voltage V₀ to theabsolute temperature T can be expressed by Equation (2) derived fromEquation (1) if it is assumed that the voltage V_(cc) of theconstant-voltage power supply 1 has no temperature dependence.

∂V₀/∂T=n R₅₁/(R₅₁+R₅₂+R₅₃)×∂V_(F)/∂T[V/° C.]  (2)

From Equation (2), it is known that the temperature characteristic(∂V₀/∂T) of the reference voltage V₀ is determined by the n diodesD₁˜D_(n), resistors R₅₁, R₅₂, and R₅₃, and (∂V_(F)/∂T). From Equation(2), it is therefore possible to obtain various combinations of n diodesD₁˜D_(n) and resistors R₅₁, R₅₂, and R₅₃ if voltage V_(cc) is fixed andthe value of the reference voltage V₀ is determined, and the temperaturecharacteristic (∂V₀/∂T) can be achieved for this number of combinations.

The number n of diodes D₁˜D_(n), however, is a discrete integer value.As a result, it is not possible to set any desired temperaturecharacteristic (∂V₀/∂T) by means of the reference voltage generatingcircuit described above.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a reference voltagegenerator circuit for generating a reference voltage that has a desiredtemperature characteristic and varies linearly relative to thetemperature.

A further object of the present invention is to provide a referencevoltage generating circuit for generating a reference voltage having anegative temperature coefficient.

A further object of the present invention is to provide a referencevoltage generating circuit for generating a reference voltage having apositive temperature coefficient.

A further object of the present invention is to provide a referencevoltage generating circuit for generating plural reference voltages eachhaving a desired temperature characteristic and varying linearlyrelative to the temperature.

A further object of the present invention is to provide a referencevoltage generating circuit for generating, in addition to a referencevoltage that has a desired temperature characteristic and varieslinearly relative to the temperature, a look-up voltage of which thetemperature characteristic is zero.

A further object of the present invention is to provide a referencevoltage generating circuit which can generate, by means of combinationwith an operational amplifier, plural reference voltages having adesired temperature characteristic and varying linearly relative to thetemperature.

In a reference voltage generating circuit according to the presentinvention, a constant-current source of which the current level flowinginto or out of a voltage divider junction varies linearly with a desiredtemperature coefficient is connected to the voltage divider junction ofthe voltage dividing circuit connected between the output terminals of aconstant-voltage power supply outputting a constant voltage, therebyoutputting the reference voltage from the voltage divider junction.

Preferably, the reference voltage is controlled to vary linearly with anegative temperature coefficient to the temperature.

Preferably, the constant-current source is made as an integratedcircuit.

Preferably, it further comprises a current mirror circuit which controlsthe current flowing through the first current path and the currentflowing through the second current path to be equal, a first transistorand a second transistor are respectively connected to the first currentpath and the second current path of the current mirror circuit, and acurrent inversely proportional to the value of the resistor connected tothe emitter of the first transistor and proportional to the temperatureis output from the voltage divider junction of the voltage dividingcircuit.

Preferably, the reference voltage is controlled to vary linearly with apositive temperature coefficient to the temperature.

Preferably, it further comprises a current mirror circuit which controlsthe current flowing through the first current path and the currentflowing through the second current path to be equal, a first transistorand a second transistor are respectively connected to the first currentpath and the second current path of the current mirror circuit, and acurrent inversely proportional to the value of the resistor connected tothe emitter of the second transistor and proportional to the temperatureis input to the voltage divider junction of the voltage divider circuit.

A reference voltage generating circuit according to the presentinvention may comprise a plurality of voltage divider circuits; acurrent mirror circuit controlling the current flowing through the firstcurrent path and the current flowing through the second current path tobe equal; a first transistor of which the emitter is connected to theother output terminal of the constant-voltage power supply; a secondtransistor of which the base is connected to the base and collector ofthe first transistor; a first resistor connected between the emitter ofthe second transistor and the other output terminal of theconstant-voltage power supply; a third and a fourth transistor; a fifthtransistor of which the base is connected to the collector of the fourthtransistor, the emitter is connected to the base of the thirdtransistor, and the collector is connected to one of the outputterminals of the constant-voltage power supply; and current extractingtransistors of which each base is connected to the emitter of the fifthtransistor, and each collector is connected to each voltage dividerjunction of the voltage divider circuits; such that a current inverselyproportional to the value of the first resistor and proportional to thetemperature is obtained from the voltage divider junction.

Preferably, the reference voltage generating circuit may obtain astandard voltage of which the temperature characteristic is zero fromthe emitter of the fifth transistor.

Preferably, the reference voltage generating circuit may connect theconstant-current source and the plural sets of voltage divider circuitsto the output of an operational amplifier, and connect the standardpower supply output from the constant-current source to the operationalamplifier.

Preferably, the constant-current source is made as an integratedcircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives and features of the present inventionwill become more apparent from the following description of a preferredembodiment thereof with reference to the accompanying drawings,throughout which like parts are designated by like reference numerals,and wherein:

FIG. 1 is a circuit diagram of a conventional reference voltagegenerating circuit.

FIG. 2 is a diagram of a reference voltage generating circuit accordingto the first embodiment of the present invention;

FIG. 3 is a circuit diagram of a reference voltage generating circuitgenerating a reference voltage having a negative temperaturecharacteristic according to the second embodiment of the presentinvention;

FIG. 4 is a circuit diagram of a reference voltage generating circuitgenerating a reference voltage having a negative temperaturecharacteristic according to the third embodiment of the presentinvention;

FIG. 5 is a circuit diagram of the constant-current source used in thereference voltage generating circuit according to the fourth embodimentof the present invention;

FIG. 6 is a circuit diagram of the reference voltage generating circuitaccording to the fourth embodiment of the present invention comprisingthe constant-current source shown in FIG. 4 for generating pluralreference voltages.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a reference voltage generating circuit according to a firstembodiment of the present invention. As shown in FIG. 2, resistors R₁and R₂ forming the voltage divider circuit are connected in seriesbetween the ground and the constant-voltage power supply 1 outputting aconstant voltage V_(cc). The constant-current source 2 is connectedbetween the ground and the voltage divider junction, which is theconnection between the resistors R₁ and R₂. The constant-current source2 varies linearly with a desired temperature characteristic the level ofthe current I₁ flowing to or from said voltage divider junction. Anoutput terminal 3 is also connected to the voltage divider junction, andthe reference voltage V₀ is output from between the output terminal 3and the ground.

If V_(cc) is the voltage of the constant-voltage power supply 1, I_(R1)is the current flowing through resistor R₁, I₁ is the current input toor from the voltage divider junction by the constant-current source 2,I_(R2) is the current flowing through resistor R₂, and V₀ is thereference voltage output from the output terminal 3 and ground, thenI_(R1)=I₁+I_(R2), V_(cc)=I_(R1)R₁+I_(R2)R₂, and V₀=I_(R2)R₂. If I_(R1)and I_(R2) are eliminated from these three Equations, the followingEquation (3) is obtained.

V₀=R₂V_(cc)/(R₁+R₂)−R₁R₂I₁/(R₁+R₂)  (3)

The temperature characteristic (∂V₀/∂T) expressed by Equation (4) belowis obtained from Equation (3).

∂V₀/∂T=−R₁R₂/(R₂+R₃)×∂I₁/∂T  (4)

As described above, because the constant-current source 2 varieslinearly with a desired value the temperature characteristic (∂I₁/∂T) ofthe current I₁ flowing to or from the voltage divider junction, thetemperature coefficient of the reference voltage V₀ output from theoutput terminal 3 can also be varied linearly with a desired temperaturecoefficient as shown by Equation (4).

Another embodiment of a reference voltage generating circuit accordingto the present invention is shown in FIG. 3. Note that like parts areidentified by the same reference numerals in FIGS. 2 and 3, andduplicated description is therefore omitted. In the reference voltagegenerating circuit shown in FIG. 3, the constant-current source 2 is anintegrated circuit comprising resistors R₃ through R₈, and bipolartransistors (simply “transistors” below) Q₁ through Q₉. In addition,resistors R₇ and R₈, and pnp-type transistors Q₄ through Q₇ form acurrent mirror circuit wherein the current I₂ flowing from the collector(first current path) of the pnp-type transistor Q₆, and the current I₃flowing from the collector (second current path) of the pnp-typetransistor Q₇, are always equal (I₂=I₃) even when the voltage V_(cc) ofthe constant-voltage power supply 1 changes.

Resistor R₇ is connected between the emitter of transistor Q₄ and theconstant-voltage power supply 1. The collector of transistor Q₄ isconnected with the emitter of transistor Q₆, and the collector oftransistor Q₆ is connected to the collector of transistor Q₂.

Resistor R₈ is connected between the emitter of transistor Q₅ and theconstant-voltage power supply 1. The collector of transistor Q₅ isconnected with the emitter of transistor Q₇, and the collector oftransistor Q₇ is connected to the collector of transistor Q₃.

The collector of transistor Q₄ is also connected to the base oftransistor Q₄ and the base of transistor Q₅, and the collector oftransistor Q₇ is also connected to the base of transistor Q₆ and thebase of transistor Q₇.

The emitter of transistor Q₂ is connected directly to ground. ResistorR₄ is connected between the emitter of transistor Q₃ and ground. Thebases of transistors Q₂, Q₃, and Q₉ are all connected to the emitter oftransistor Q₁. Resistor R₅ is connected between the emitter oftransistor Q₁ and ground. Transistor Q₁ compensates the base current oftransistors Q₂ and Q₃ to improve the precision of constant-currentgeneration by the transistors Q₂ and Q₃.

The base of transistor Q₈ is also connected to the collector oftransistor Q₃. Transistor Q₈ comprises a circuit for activating theconstant-current source 2 with the collector of transistor Q₈ connectedto the constant-voltage power supply 1, and a resistor R₆ connectedbetween the emitter thereof and ground. A resistor R₃ is connectedbetween the emitter of the transistor Q₉ and ground, and the collectorof transistor Q₉ is connected to the junction (voltage divider junction)between resistor R₁ and resistor R₂. Transistor Q₉ and resistor R₃ arepart of the integrated circuit, and have the same transistor size andresistance as transistor Q₃ and resistor R₄.

In the reference voltage generating circuit shown in FIG. 3, thereference voltage V₀ output from between the ground and the outputterminal 3 can be obtained as follows. Equation (5) shown below isobtained where V_(BE2) is the voltage between the base and emitter ofthe transistor Q₂, V_(BE3) is the voltage between the base and emitterof the transistor Q₃, and V_(R4) is the voltage drop in resistor R₄.

V_(BE2)=V_(BE3)+V_(R4)  (5)

If the emitter size ratio of transistors Q₂ and Q₃ is 1:N, thesaturation current of transistor Q₂ is I_(S), and V_(T)=kT/q (where q isthe electron charge, k is Boltzmann's constant, and T is the absolutetemperature), the base-emitter voltage V_(BE2) of transistor Q₂ isexpressed as V_(BE2)=V_(T)1n(I₂/I_(s)) based on Shockley's Equation, andthe base-emitter voltage V_(BE3) of transistor Q₃ is expressed asV_(BE3)=V_(T)1n(I₃/NI_(s)). By substituting these values into Equation(5), the following Equation (6) is obtained because I₂=I₃.

V_(R4)=V_(T)1n(N)  (6)

From Equation (6), I₂=I₃ can be expressed by the following Equation (7).

I₂=I₃=V_(R4)/R₄

 =(V_(T)/R₄)1n(N)  (7)

Because V_(T)=kT/q, V_(T) is proportional to the absolute temperature T,currents I₂ and I₃ are therefore also proportional to the absolutetemperature T based on Equation (7). Because transistor Q₉ and resistorR₃ have the same transistor size and resistance as transistor Q₃ andresistor R₄ in the integrated circuit, the collector current I₁ oftransistor Q₉ has the relationship I₁=I₂=I₃, is proportional to theabsolute temperature T, and is expressed by the following Equation (8).

I₁=V_(R4)/R₄

 =(V_(T)/R₄)1n(N)

 =(kT/qR₄)1n(N)  (8)

From Equation (8), the reference voltage V₀ output from the outputterminal 3 of the reference voltage generating circuit in FIG. 3 can beexpressed by the following Equation (9).

V₀=R₂V_(cc)/(R₁+R₂)−R₁R₂I₁/(R₁+R₂)

 =R₂V_(cc)/(R₁+R₂)−(kT/qR₄)1n(N)R₁R₂/(R₁+R₂)  (9)

Therefore, the temperature characteristic (∂V₀/∂T) of the referencevoltage V₀ output from the output terminal 3 can be expressed by thefollowing Equation (10).

∂V₀/∂T=−(k/qR₄)1n(N)R₁R₂/(R₁+R₂)(=constant)  (10)

As shown by Equation (10), the temperature characteristic (∂V₀/∂T) ofthe reference voltage V₀ is inversely proportional to resistor R₄, andreference voltage V₀ varies linearly with a negative temperaturecoefficient to the absolute temperature T. Thus, the reference voltageV₀ can be varied linearly relative to the temperature with a desirednegative temperature coefficient by selecting resistor R₄.

It is to be noted that a reference voltage generating circuit operatingidentically to that described above can be achieved by shorting resistorR₃ connected between the ground and the emitter of transistor Q₉ in FIG.2, and making transistor Q₉ the same size as transistor Q₂ in the aboveembodiment.

A third embodiment of a reference voltage generating circuit accordingto the present invention is shown in FIG. 4. Note that like parts areidentified by like reference numerals in FIGS. 2 and 4, and duplicateddescription is therefore omitted. In the reference voltage generatingcircuit, the constant-current source 2 is an integrated circuitcomprising resistors R₉ and R₁₀, npn-type transistors Q₁₀, Q₁₁, and Q₁₂,and pnp-type transistors Q₁₃, Q₁₄, and Q₁₅. Transistors Q₁₃, Q₁₄, andQ₁₅ form a current mirror circuit constituted such that the current I₄flowing to the collector of transistor Q₁₀, and the current I₅ flowingto the collector of transistor Q₁₁, are always equal (I₄=I₅) even whenthe voltage V_(cc) of the constant-voltage power supply 1 changes. Inaddition, resistor R₁₀ and transistor Q₁₂ constitute a starting circuitfor activating the current mirror circuit. The emitter of transistor Q₁₃is connected to the constant-voltage power supply 1, and the collectorthereof is connected to the collector of transistor Q₁₀. The emitter oftransistor Q₁₄ is connected to the constant-voltage power supply 1, andthe collector thereof is connected to the collector of transistor Q₁₁.The base of transistor Q₁₃ and the base of transistor Q₁₄ are mutuallyconnected, and the collector of transistor Q₁₀ is connected to the baseof transistor Q₁₀ and the base of transistor Q₁₁. The emitter oftransistor Q₁₅ is connected to the base of both transistors Q₁₃ and Q₁₄,the base of transistor Q₁₅ is connected to the collector of transistorQ₁₄, and the collector of transistor Q₁₅ is connected to the ground. Inaddition, resistor R₁₀ is connected between the ground and the emitterof transistor Q₁₂, the base of transistor Q₁₂ is connected to theemitter of transistor Q₁₅, and the collector of transistor Q₁₂ isconnected to the constant-voltage power supply 1.

The emitter of transistor Q₁₀ is connected to the voltage dividerjunction of resistors R₁ and R₂ forming the voltage divider circuit.Resistor R₉ is also connected between the voltage divider junction andthe emitter of transistor Q₁₁.

In the reference voltage generating circuit shown in FIG. 4, referencevoltage V₀ output from between the ground and the output terminal 3 canbe obtained as follows. Equation (11) given below is obtained whereV_(BE10) is the voltage between the base and emitter of the transistorQ₁₀, V_(BE11) is the voltage between the base and emitter of thetransistor Q₁₁, and V_(R9) is the voltage drop of resistor R₉ in thereference voltage generating circuit shown in FIG. 4.

V_(BE10)=V_(BE11)+V_(R9)  (11)

If the emitter size ratio of transistors Q₁₀ and Q₁₁ is 1:N, thesaturation current of transistor Q₁₀ is I_(S), and V_(T)=kT/q (where qis the electron charge, k is Boltzmann's constant, and T is the absolutetemperature), the base-emitter voltage V_(BE10) of transistor Q₁₀ isexpressed as V_(BE10)=V_(T)1n(I₄/I_(s)) based on Shockley's Equation,and the base-emitter voltage V_(BE11) of transistor Q₁₁ is expressed asV_(BE11)=V_(T)1n(I₁₁/NI_(s)). By substituting these values into Equation(11), the following Equation (12) is obtained because I₄=I₅.

V_(R9)=V_(T)1n(N)  (12)

From Equation (12), I₄=I₅ can be expressed by the following Equation(13).

I₄=I₅=V_(R9)/R₉

 =(V_(T)/R₉)1n(N)  (13)

However, because the current I₁ input to the voltage divider junction ofresistors R₁ and R₂ is the sum of current I₄ and current I₅,I₁=−(I₄+I₅), and current I₁ can be expressed by Equation (14) based onEquation (13).

I₁=−2(V_(T)/R₉)1n(N)  (14)

From Equation (14), the reference voltage V₀ output from the outputterminal 3 of the reference voltage generating circuit in FIG. 4 can beexpressed by the following Equation (15).

V₀=R₂V_(cc)/(R₁+R₂)−R₁R₂I₁/(R₁+R₂)

 =R₂V_(cc)/(R₁+R₂)+2(kt/qR₉)1n(N)R₁R₂/(R₁+R₂)  (15)

Therefore, the temperature characteristic (∂V₀/∂T) of the referencevoltage V₀ output from the output terminal 3 can be expressed by thefollowing Equation (16).

∂V₀/∂T=2(kt/qR₉)1n(N)R₁R₂/(R₁+R₂)(=constant)  (16)

As shown by Equation (16), the temperature characteristic (∂V₀/∂T) ofthe reference voltage V₀ is inversely proportional to the value ofresistor R₉, and reference voltage V₀ varies linearly with a positivetemperature coefficient to the absolute temperature T. Thus, thereference voltage V₀ can be varied linearly relative to the temperaturewith a desired positive temperature coefficient by selecting resistorR₉.

A fourth embodiment of a reference voltage generating circuit accordingto the present invention is shown in FIGS. 5 and 6. This embodiment is acircuit for generating plural reference voltages; FIG. 5 shows theintegrated constant-current supply circuit 6 for generatingconstant-currents I₄₁ through I_(4m), and FIG. 6 shows the specificcircuitry of a reference voltage generating circuit comprising theconstant-current supply circuit 6. The constant-current supply circuit 6in FIG. 5 comprises resistors R₂₀through R₂₆, resistors R₃₁ throughR_(3m), transistors Q₂₀ through Q₂₈, and transistors Q₃₁ through Q_(3m).Transistors Q₂₃ and Q₂₄, and resistors R₂₁ and R₂₂ form a current mirrorcircuit constituted such that the collector current of transistor Q₂₂and the collector current of transistor Q₂₈ are always equal even whenthe voltage V_(cc) of the constant-voltage power supply 1 changes. Inaddition, resistors R₂₄ and R₂₅ maintain a constant ratio between thecollector currents of transistors Q₂₆ and Q₂₇. Resistor R₂₁ is connectedbetween the emitter of transistor Q₂₃ and the constant-voltage powersupply 1. The collector of transistor Q₂₃ and the collector oftransistor Q₂₂ are mutually connected, and resistor R₂₃ is connectedbetween ground and the emitter of transistor Q₂₂. Resistor R₂₂ isconnected between the emitter of transistor Q₂₄ and the constant-voltagepower supply 1. The collector of transistor Q₂₄ and the collector oftransistor Q₂₈ are mutually connected, and the emitter of transistor Q₂₈is connected to ground. The collector of transistor Q₂₂ is connected tothe base of transistor Q₂₃ and the base of transistor Q₂₄.

The base of transistor Q₂₅ is connected to the collector of transistorQ₂₄, and the collector of transistor Q₂₅ is connected to theconstant-voltage power supply 1. The emitter of transistor Q₂₅ isconnected to the base of transistor Q₂₂, and to the bases of transistorsQ₃₁ through Q_(3m). The emitter of transistor Q₂₆ is connected toground, and resistor R₂₄ is connected between the collector oftransistor Q₂₆ and the emitter of transistor Q₂₅. Resistor R₂₆ isconnected between ground and the emitter of transistor Q₂₇, and resistorR₂₅ is connected between the collector of transistor Q₂₇ and the emitterof transistor Q₂₅. The collector and emitter of transistor Q₂₆ aremutually connected. The base of transistor Q₂₀ and the base oftransistor Q₂₁ are mutually connected. The base and collector oftransistor Q₂₀ are mutually connected, the emitter thereof is connectedto ground, and resistor R₂₀ is connected between the collector oftransistor Q₂₀ and the constant-voltage power supply 1. The emitter oftransistor Q₂₁ is connected to the emitter of transistor Q₂₂, and thecollector thereof is connected to the collector of transistor Q₂₂.

The base of each of transistors Q₃₁ through Q_(3m) is connected to theemitter of transistor Q₂₅, and resistors R₃₁ through R_(3m) arerespectively connected between ground and the emitter of each oftransistors Q₃₁ through Q_(3m). The reference voltage output terminal 7outputting the reference voltage V_(REF) is connected to the emitter oftransistor Q₂₅.

In the reference voltage generating circuit shown in FIG. 5, currentsI₄₁ through I_(4m) flowing to the corresponding collectors oftransistors Q₃₁ through Q_(3m) can be obtained as described below.Because the collector current ratio of transistors Q₂₆ and Q₂₇ isdetermined by resistors R₂₄ and R₂₅ as already described, the collectorcurrent I₆ of transistor Q₂₆ and the collector current I₇ of transistorQ₂₇ will become equal if the base current of transistor Q₂₈ is ignoredwhen the values of resistor R₂₄ and resistor R₂₅ are equal.

Equation (17) given below is obtained where V_(BE26) is the voltagebetween the base and emitter of the transistor Q₂₆, V_(BE27) is thevoltage between the base and emitter of transistor Q₂₇, and V_(R26) isthe voltage drop in resistor R₂₆ of the circuit shown in FIG. 5.

V_(BE26)=V_(BE27)+V_(R26)  (17)

If the emitter size ratio of transistors Q₂₆ and Q₂₇ is 1:N, thesaturation current of transistor Q₂₆ is I_(S), and V_(T)=kT/q (where qis the electron charge, k is Boltzmann's constant, and T is the absolutetemperature), the base-emitter voltage V_(BE26) of transistor Q₂₆ isexpressed as V_(BE26)=V_(T)1n(I₆/I_(s)) based on Shockley's Equation,and the base-emitter voltage V_(BE27) of transistor Q₂₇ is expressed asV_(BE27)=V_(T)1n(I₇/NI_(s)). By substituting these values into Equation(17), the following Equation (18) is obtained because I₆=I₇.

V_(R26)=V_(T)1n(N)  (18)

From Equation (18), I₆=I₇ can be expressed by the following Equation(19).

I₆=I₇=V_(R26)/R₂₆

 =(V_(T)/R₂₆)1n(N)  (19)

Because V_(T)=kT/q, V_(T) is proportional to the absolute temperature T,currents I₆ and I₇ are therefore also proportional to the absolutetemperature T based on Equation (19). If transistor Q₂₂ and resistor R₂₃are made from the same devices as transistor Q₂₆ and resistor R₂₄, thecurrent input to transistor Q₂₂ will be equal to the current I₆described above. Similarly, if transistors Q₃₁ through Q_(3m) andresistors R₃₁ through R_(3m) are likewise made from the same devices astransistor Q₂₆ and resistor R₂₄, I₄₁= . . . I_(4m)=I₆. It is thereforeknown that currents I₄₁ through I_(4m) also vary proportionally to theabsolute temperature T.

The reference voltage V_(REF) (expressed as V_(BG)) output from thereference voltage output terminal 7 is the sum of the base-emitterforward voltage drop V_(BE28) of transistor Q₂₈ and the voltage drop ofresistor R₂₅, and is obtained by Equation (20) below.

V_(BE)=V_(BE28)+R₂₅I₇

 =V_(BE28)+(R₂₅V_(T)/R₂₆)1n(N)  (20)

The temperature characteristic of V_(BG) will be zero (0) if the circuitconstant is set so that the temperature characteristic of thebase-emitter forward voltage drop V_(BE28) of transistor Q₂₈ and thetemperature characteristic of (R₂₅V_(T)/R₂₆)1n(N) are mutuallycanceling. In this case, a reference voltage V_(REF) with a temperaturecharacteristic of zero can be obtained from the reference voltage outputterminal 7. It is to be noted that when the temperature characteristicof V_(BG) in the circuit in FIG. 5 is zero (0), V_(BG) is called theband gap voltage, and is usually 1.25 volts.

As shown in FIG. 6, the constant-current supply circuit 6 describedabove and resistors R₃₁ and R₃₂ are connected between the ground and theoutput terminal of the operational amplifier 5 comprising theconstant-voltage power supply 1, which stabilizes and outputs theunstabilized power supply voltage of the power supply 4. The referencevoltage V_(REF) generated by the constant-current supply circuit 6 issupplied to the noninverting input of the operational amplifier 5, andis connected to the voltage divider junction of voltage dividerresistors R₃₁ and R₂₃ serially connected between ground and the outputterminal of the operational amplifier 5. The collectors (see FIG. 5) ofthe transistors Q₃₁ through Q_(3m) of the constant-current supplycircuit 6 are respectively connected to the voltage divider junction ofvoltage divider resistors R₁₋₁ and R₂₋₁, and voltage divider resistorsR_(1-m) and R_(2-m), serially connected between the ground and theoutput terminal of the operational amplifier 5. By using theconstant-current supply circuit 6 in FIG. 5, it is possible to generateplural reference voltages each having a temperature characteristicvarying linearly relative to temperature by simply combining theoperational amplifier 5 with the constant-current supply circuit 6, andwithout using Zener diodes or other devices generating the referencevoltage V_(REF). It is also possible by means of the circuitry of theconstant-current supply 6 to increase the voltage drop generated at theresistors R₃₁ through R_(3m) determining the constant-current value, andthis circuitry is suited to constituting plural constant-currentsupplies because error in the constant-current value caused by degradedrelativity between transistor Q₂₂ and the npn-type transistors Q₃₁through Q_(3m) can be reduced.

An advantage of the present invention is that a reference voltagevarying linearly with a desired temperature coefficient can be obtainedfrom the voltage divider junction of the voltage divider circuit becausethe constant-current source linearly varies the current level flowing toor from the voltage divider junction of the voltage divider circuit witha desired temperature coefficient.

Another advantage of the present invention is that a reference voltagevarying linearly with a desired negative temperature coefficient can beobtained from the voltage divider junction of the voltage dividercircuit because the constant-current source varies the current obtainedfrom the voltage divider junction of the voltage divider circuitlinearly with respect to temperature with a desired temperaturecoefficient.

A further advantage of the invention is that a reference voltage varyinglinearly with a desired negative temperature coefficient can be obtainedfrom the voltage divider junction of the voltage divider circuit byselecting the value of the resistor connected to the emitter of thesecond transistor because the current flowing from the voltage dividerjunction of the voltage divider circuit is proportional to temperatureand inversely proportional to the value of the resistor connected to theemitter of the second transistor.

A still further advantage of the present invention is that a referencevoltage varying linearly with a desired positive temperature coefficientcan be obtained from the voltage divider junction of the voltage dividercircuit because the constant-current source varies the current input tothe voltage divider junction of the voltage divider circuit linearlywith respect to temperature with a desired temperature coefficient.

A still further advantage of the present invention is that a referencevoltage varying linearly with a desired positive temperature coefficientcan be obtained from the voltage divider junction of the voltage dividercircuit by selecting the value of the resistor connected to the emitterof the second transistor because the current extracting transistorfunctions to input to the voltage divider junction of the resistor-typevoltage dividing circuit a current proportional to temperature andinversely proportional to the value of the resistor connected to theemitter of the second transistor connected to the second current path ofthe first and second current paths of the current mirror circuit.

A still further advantage of the present invention is that pluralreference voltages each varying linearly with a desired negativetemperature coefficient can be obtained from the voltage dividerjunction of each voltage divider circuit by selecting the value of afirst resistor because the current extracting transistor functions toextract from each voltage divider junction of plural voltage dividercircuits a current proportional to temperature and inverselyproportional to the value of the first resistor, which is connectedbetween the other output terminal of the constant-voltage power supplyand the emitter of the second transistor of which the base is connectedto the base and the collector of a first transistor of which the emitteris connected to the other output terminal of the constant-voltage powersupply.

Because a standard voltage with a temperature characteristic of zero isoutput from the emitter of a fifth transistor, this standard voltage canbe used as the standard voltage of the constant-voltage power supply.

Because an operational amplifier controls its output voltage to maintaina constant difference between said output voltage and the standardvoltage, the operational amplifier functions as the constant-voltagepower supply for connecting the constant-current source, and thereference voltage generating circuit can be simply constituted.

Because the thermal coupling between components is improved and thermalresponse is also improved by constituting the constant-current source bymeans of an integrated circuit, charging optimized to the temperature ofthe battery can be achieved by inclusion in the battery chargingapparatus.

Although the present invention has been described in relation toparticular embodiments and other uses will become apparent to thoseskilled in the art. It is preferred, therefore, that the presentinvention be limited not by the specific disclosure herein, but only bythe appended claims.

What is claimed is:
 1. A reference voltage generating circuit forgenerating a reference voltage that changes linearly with temperature,the reference voltage generating circuit comprising: a constant-voltagepower supply for outputting a constant voltage and having first andsecond output terminals; a voltage divider circuit connected between thefirst and second output terminals; and a constant-current sourceconnected to a voltage divider junction of said voltage divider circuitfor linearly changing with temperature current flowing into or out ofthe voltage divider junction, a reference voltage being output from saidvoltage divider junction, said constant-current source including acurrent mirror circuit comprising: a first current path and a secondcurrent path established by a connection to the second output terminalfor equalizing respective currents flowing through the first and secondcurrent paths, a first transistor having a base and connected betweenthe first current path and the second output terminal, a secondtransistor having an emitter and a collector, the collector beingconnected in the second current path, a resistor having a resistance andconnected between the emitter of said second transistor and the secondoutput terminal, and a current extracting transistor having a baseconnected to the base of said first transistor and to the base of saidsecond transistor, and a collector connected to the voltage dividerjunction, for sensing current flow at the voltage divider junction,wherein a current flow inversely proportional to the resistance of saidresistor and proportional to temperature is obtained at the voltagedivider junction.
 2. The reference voltage generating circuit accordingto claim 1, wherein the reference voltage changes inversely withtemperature changes.
 3. The reference voltage generating circuitaccording to claim 1, wherein said constant-current source comprises anintegrated circuit.
 4. The reference voltage generating circuitaccording to claim 5, wherein the reference voltage changes directlywith temperature changes.
 5. A reference voltage generating circuit forgenerating a reference voltage that changes linearly with temperature,the reference voltage generating circuit comprising: a constant-voltagepower supply for outputting a constant voltage and having first andsecond output terminals; a voltage divider circuit connected between thefirst and second output terminals; and a constant-current sourceconnected to a voltage divider junction of said voltage divider circuitfor linearly changing with temperature current flowing into or out ofthe voltage divider junction, a reference voltage being output from saidvoltage divider junction, said constant-current source including acurrent mirror circuit comprising: a first current path and a secondcurrent path established by a connection to the first output terminalfor equalizing currents flowing through the first and second currentpaths, a first transistor connected between the first current path andthe voltage divider junction, a second transistor having an emitter anda collector, the collector being connected in the second current path,and a resistor having a resistance and connected between the emitter ofsaid second transistor and the voltage divider junction, wherein acurrent flow inversely proportional to the resistance of the resistorand proportional to temperature is obtained at the voltage dividerjunction.
 6. A reference voltage generating circuit for generating areference voltage having a negative temperature coefficient, thereference voltage generating circuit comprising: a constant-voltagepower supply for outputting a constant voltage and having first andsecond terminals; a plurality of voltage divider circuits connectedbetween the first and second output terminals of the constant-voltagepower supply; and a constant-current source, said constant-currentincluding: a current mirror circuit comprising a first current path anda second current path established by a connection with the second outputterminal for equalizing currents flowing through the first and secondcurrent paths; a first transistor having a base, a collector, and anemitter, the emitter being connected to the second output terminal ofsaid constant-voltage power supply; a second transistor having anemitter and a base, the base of said second transistor being connectedto the base and the collector of said first transistor; a first resistorhaving a resistance and connected between the emitter of said secondtransistor and the second output terminal; a third transistor having anemitter and a collector, the collector of said third transistor beingconnected to the first current path; a second resistor connected betweenthe emitter of said third transistor and the second output terminal; afourth transistor having a collector and an emitter, the collector ofthe fourth transistor being connected in the second current path, andthe emitter of said fourth transistor being connected to the secondoutput terminal; a fifth transistor having an emitter, a collector, anda base, the base of said fifth transistor being connected to thecollector of said fourth transistor, the emitter of said fifthtransistor being connected to the base of said third transistor, and thecollector of said fifth transistor being connected to the first outputterminal; a third resistor connected between the emitter of said fifthtransistor and the collector of said first transistor; a fourth resistorconnected between the emitter of said fifth transistor and the collectorof said second transistor; and sixth transistors, each sixth transistorhaving a collector, an emitter, and a base, each base of said sixthtransistors being connected to the emitter of said fifth transistor,each emitter of said sixth transistors of said sixth transistors beingconnected to the second output terminal, and each collector of saidsixth transistors being connected to a voltage divider junction of arespective one of the voltage divider circuits, wherein a currentinversely proportional to the resistance of said first resistor andproportional to temperature is obtained at the voltage divider junction.7. The reference voltage generating circuit according to claim 6,wherein the emitter of said fifth transistor supplies a voltage thatdoes not vary with temperature.
 8. The reference voltage generatingcircuit according to claim 8 wherein the constant-voltage power supplycomprises: an operational amplifier having an input and an output, and afeedback resistor connected to the output of said operational amplifierto feedback changes in the output to the input, wherein saidconstant-current source and said voltage divider circuits are connectedto the output of said operational amplifier, and the voltage output fromsaid constant-current source is connected to the input of saidoperational amplifier.
 9. The reference voltage generating circuitaccording to claim 8, wherein said constant-current source comprisesintegrated circuit.
 10. A reference voltage generating circuit forgenerating a reference voltage that changes linearly with thetemperature, the reference voltage generating circuit comprising: aconstant-voltage power supply for outputting a constant voltage andhaving first and second output terminals; a voltage divider circuitconnected between the first and second output terminals; and aconstant-current source connected to a voltage divider junction of saidvoltage divider circuit for linearly changing with temperature currentflowing into or out of said voltage divider junction, a referencevoltage linearly changing with temperature being output from saidvoltage divider junction.
 11. The reference voltage generating circuitaccording to claim 10, wherein the reference voltage changes inverselywith temperature changes.
 12. The reference voltage generating circuitaccording to claim 10, wherein said constant-current source comprises anintegrated circuit.